Have a pc power supply that's got a 12v regulated output that I want to put into an lm217t to get an adjustable output. I have a B1k variable resistor to hand so would prefer to use that to get it to work and want as close as possible to 0v-12v variable output

You will not get the full 12V output using the LM317 family of adjustable regulators when the supply is only 12V.
There will be some drop across the regulator that will vary with current drawn.
Review the data sheet for the actual (expected) drop vs current.

You will not get the full 12V output using the LM317 family of adjustable regulators when the supply is only 12V.
There will be some drop across the regulator that will vary with current drawn.
Review the data sheet for the actual (expected) drop vs current.

see jpg for typical application.

(capacitors for stability not shown)

Yeh I know I won't get the full 12v and that's fine just wanted the best optimization for this little circuit as possible

How many amps does the lm217t supply?

What caps would you recommend? Or do I not need them as it is a fully regulated power supply...

Yes you can use your 1kVR to control the output voltage of your supply.
The range of adjustment for a 12Vdc input will be ~ 1.25Vdc to 10Vdc.

The dissipation when set to low voltage will severely limit the output current.

There are other Threads showing better ways to do this.

I mention what I have done.
2.75A (secondhand Ebay) Variac feeding a 100VA 25+25Vac transformer. Output is variable from 0+0Vac to 25+25Vac. This is fitted into the box holding my bulb tester and my mains output from the same Variac.
Two pair of 4mm banana sockets feed that variable AC into a pair of PSUs to give me +-0Vdc to +-35Vdc @ ~1Adc continuous and many Apk to power most of what I build.
I also have an Ebay second hand dual output Lab supply that does 0 to 30Vdc @ <=3.1Adc continuous, twice over, with variable current limiters and ability to parallel or series connect the two independent outputs.

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regards Andrew T.
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If the 12 volt supply is regulated, u need not go for an LM317. Just construct a variable voltage circuit using single transistor and couple of parts. Added advantage is that u will get a bigger span than the LM317.